A neurobiological understanding of games has at its core the dopaminergic reward system . The nucleus accumbens, which is also dubbed the pleasure centre of the brain, is currently understood as the critical brain region associated with the neurotransmitter dopamine, which in turn is implicated in habit formation, reward-seeking behaviours and addiction. The above video shows a speech from Robert Sapolsky, Professor of Neurology and Neurological Sciences at Stanford University, where he discusses his research concerning dopamine release in the brain when rewarded and when anticipating a reward.
Something gambling machine designers and many game designers probably intuitively know is that near-misses enhance gambling motivation. The release of dopamine tops out with near misses in gaming. Now, Prof. Sapolsky’s point in his slides is that in his studies when participants (or monkeys) only get rewarded 50% of the time that they put work into a task, their dopamine peaks much higher than if they are rewarded every time (and the spike level is raised before the reward is received). He transfers his points to everyday examples of people trying to achieve things that have a delayed reward (such as studying, raising money to get a better house, moving into your favourite retirement home, or finally being famous post death). Not only is this interesting in understanding what drives humans to try and accomplish basically any task, but also for game designers to use chance to their advantage in making games ever more exciting and motivate us to play (think epic items/armour in World of Warcraft).
If we strike a good balance between giving out a reward for a task and withholding the reward, the fuzziness of criteria for decision-making will likely increase personal motivation. This could go further, too, in terms of value assigned to rewards, things that are harder to achieve might gain an increased value if they are not available all of the time. Take a high-level raid in World of Warcraft for example, where high-tier items will drop after defeating very difficult opponents, but not every time enough items for the whole raiding party will drop, so that people will stay motivated to do this raid again. The randomness of unique items drops is one of the primary replay factors for games like Diablo and Torchlight, where level content is reused, but players stay motivated with the anticipation of gearing up with more high level items.
Thinking about this motivational dynamic of randomly achievable rewards, the question is what happens once all content is there, once the endgame is done? This is where the metagame of achievements and trophies, and in a broader context, social recognition comes into play. Once we have all the rewards a program or task can give us, we would like this work to increase our social standing on a grander scale, to be recognized by peers and have material to increase and enhance our social interactions. This also ties strongly into social motivations for ethical behaviour and religious beliefs, as Sapolsky jokingly puts it, there might be a reward in the afterlife waiting for us.